Transmission control circuits



Aug. 14, 1934. A. c. NQRWINE TRANSMISSION CONTROL CIRCUITS Filed April 25, 1951 A TTORNEY Patented Aug. 14, 1934 1,970,334 TRANSMISSION CONTROL CIRCUITS Andrew CfNorwina-Passaic, N.' J'., assigner to Bell Telephone Laboratories, Incorporated, New vYork, N. Y., a corporation of New York Application April 25, 193i, sei-iai No. 532,722 Y 12 cian-as, (o1. irc-170) This invention relates to two-wayl signaling systems and particularly to'means for controlling transmission in such systems.

The object ofthe invention is to improve theV operation of a signal transmissionsystem including therein circuit-control means operated under control of the signals transmitted over the system.

The invention is especially applicable to a system in which two-way' transmission of signalingencrgy is required and including as a link therein .a two-way cable of highy attenuation and long transmission time, such as a long, deep sea submarine telephone cable. Signal-controlled, circuit-control apparatus are usually Vassociated with the transmittingand receiving circuits at each terminal of the cable in such asystem to insure that the system is operative to transmit in only one direction at a time, so as eiectively to prevent singing and to suppress' echoes. This is usually accomplished by making the circuitcontrol apparatus at each terminal responsive to transmission in one direction to render the circuits thereat operative to transmit in that direction and eieotively Vto disable the circuits for transmission in the oppositefdirection. v

The relatively long time (as compared to radio circuits, for example) required for transmitting signals over a cable in such a system; greatly in-` creases the chance .that two subscribers at opposite ends of the system may both start talking within a' time interval equal to the overall'transmission time of the cable,"resulting in 'each'subscriber seizing control of the circuit-control apparatus at his 'own terminal and locking out the transmission of the other subscriber.

Circuits have heretofore been designed to avoid the above mentioned difficulty by giving complete control of the circuit-control apparatus at both ends of the system to that subscriber who iirst starts talking. This is accomplished incertain of these circuits by providing meansV at each terminal under' control of the speech currents initiated thereat for transmitting over the systern along with, or in advance of, the speech waves,v one or more control waves tooperate the circuit-control apparatus in their path. The control wave iirst sent out determines which terminal or talker gets control of the system;

in one aspect the invention relates to' a circuit of the above mentioned type in which break-insby the provision of an auxiliary control circuit at each terminal responsive to the initiation of speech waves thereat for disabling the receiving control .circuit at that terminal substantially at the same time as the arrival of the control Waves at the other terminal, and for reestablishing said control circuit substantially simultaneously with the cessation of said speech waves, thus enabling a listener to seize control of the talking circuits of the system at a pause by the talker for a time considerably less than the overall transmission time of the cable. y s

VAnother Vfeature is that oi utilising for controllingv circuit-control apparatus at each terminal ci the system a beat frequency produced by combining two control frequencies or other means requiring for operation both or all control frequencies transmitted from the other terminal under control of the speech waves initiated thereat. The control frequencies are .dierent foreach terminal, and are preferably chosen so 'that they are normally not to be found simultaneously inspeech, in their harmonics or diiference tones. This method of control lessens the chance of false operation oi the circuit-control apparatus..

l The exact nature of the invention and its advantages will oe-clear from the following detailed zlesoriptlonV thereof when read in connection with the acc mpanying drawing, Fig. l of ch shows diagrammatically a portion of one teiminaiof asubmarine telephone cable system embodying the invention, and Fig. A2 shows an alternative modification of a portionof the circuit of Fig. l.

v For convenience the invention'will be described as applied to a system for the two-Way transmission of speech waves, but it is to be understood that the principles of the invention apply equally well to systems for the two-way transmission of signal waves in general of frequencies within or outside the speech frequency range, such as waves representing speech, entertainment procomprising music as well as speech, telegraph waves, etc.

' Since the terminal stations at each end of the cable inthe system to be described may be substantially identical construction except for changes in the design of the control wave generators and the filters usedvth'erein, necessitated by the fact' that the values of the frequencies of 'the control waves transmitted from the two terminals arediiferwent, it has been deemed sufficient to illustrate only one terminal of the cable system.

Fig 1 of the drawing illustrates the west terlninal circuit for a two-way telephone cable system. This west cable terminal circuit comprises a transmitting-amplifying path TA having its output connected directly to the west end of the submarine telephone cable TC, and a receivingamplifying path RA having its input connected directly to the west end of the submarine telephone cable TC. The input of the transmitting path TA and the output of the receiving path RA may be connected by means of a three-winding transformer or hybrid coil, or other suitable ineans in energy transmitting relation with `a.

circuit associated with a west subscriber, (not shown), such as a toll telephonecircu-it, and .in substantially conjugate relation with each other.

rhe transmitting-amplifying Ypath 'TA comprises reading in order from west to east, the one-way amplifying device 1 and the one-way amplifying device 2. The receiving-amplifying path RA comprises reading in order from east to west, the one-way amplifying device 3 and the filter 4.. i

Connected across the transmitting path 'IA in the input of the one-way amplifying device i is the input of a control circuit 5 comprising the wave-controlled, relay-controlling device 6 and the windings of the mechanical relays 7 to 10 connected in parallel across the output of the device 6. The source 11 of alternating current control waves of frequencies f1 and the source 12 of alternating current control waves ef `frequencies f2 are normally connected across the path TA between amplifying devices 1 and 2 therein through the band-pass filter 13.

Connected across the input of the receiving path RA `between the input of the receiving-amplifying device 3 and the west end of the cable TC is the receiving control circuit 14 comprising the selective network 15, the amplifier-detector 16, the band-pass Ynlter 17, the wave-controlled, relay-controlling device 18, and-the windings of the mechanical relays 19 and 20y connected in parallel across the output ofthe device 18. Connected across the receiving path RA between the output of the amplifying device 3 and the filter 4 therein is the input 'lof asecondreceiving control circuit 2l comprising the wave-controlled, relay-controlling `device 22, and the windings of the 'mechanical relays 19 and 20 connected in parallel across the output of the device 22.

In the output of the transmitting path TA between the output of the amplifying device 2 therein and the west end of the cable TC are the normally open switch contacts 23 adapted to be closed by operation of relay 8 to connect the output of the transmitting-amplifying device 2 to the cable TC. Also in the transmitting path TA in front of the point of connection of the control circuit 5 thereto are the normally closed switch contacts 24 adapted to be opened by operation of relay 19 to prevent transmission of speech waves to the input of the control circuit 5.

in the input of the receiving path RA between 'the point of connection of the receiving control circuit 14 thereto and the input of the arnplii'ying device 3 arethe normally closed switch contacts 37 adapted to be opened by operation of relay 7 to disconnect the receiving-amplifying device 3 from the cable .'IC. v

in the circuit connecting the kband-pass filter 13 vto the transmitting path TA are the norinally closed 'switch contacts 25 adapted to be opened by operation of relay 9 to disconnect the control wave sources 11"and 12 and the lter 13 from the transmitting path TA. In the control circuit 5 between the output of the control device 6 and the winding of relay 10 are the normally closed switch contacts 26 adapted to be opened by operation of relay 20 to disable the input to relay 10. In the receiving control circuit 14 between the selective network l5 and the amplifier-detector 16 are the normally closed switch contacts 27 adapted to be opened by operation Vof relay 10.

The wave-controlled, relay-controlling devices 6,18 and 22, and the corresponding devices at the east cable terminal station, may be vacuum tube, amplifier-rectifier devices of the type well known in the art, or any other devices which will respond to alternating current waves impressed on their inputs to control the operation of circuit-control devices, such as mechanical relays in their outputs. The amplier-detector 16 may be of the vacuum tube type well known in the art.

The selective network 15 and the corresponding network at the eaststation may be a tuned circuit or a band-,pass filter. The former is designed to select 4the control. wave frequencies f3 and f4 transmitted from the east station and the latter to select the control wave frequencies f1 and fz transmitted from the west station. i

The filters 4, 13 and 17 at the west cable .terminal station and the corresponding filters at the east cable terminal station may be of the bandpass type disclosed in United States patents to Campbell, Nos. 1,227,113 and 1,227,114 issued May 22, 1917. The filter 17 is designed to select the beat frequencyfi-fa, or any other beat frequency produced by coinbinationfof f3 and f4 where Vf3 and f4 are the control wave frequencies generated by the control wave-sources at the east cable terminal station corresponding to the sources 11 and l2 at the west cable terminal station, .and to suppress other frequencies. The lter at the east station corresponding to filter 17 is designed to select the beat frequency frz-f1, or other combination, where f1 and fzare the control wave frequenciestransmitted from thev west station. The bandepass lter 4 in the receiving path RA at the west cable terminal station is designed to suppress the control wavefrequencies f3 and f4 transmittedfromthe east cable terminal station while transmitting other frequencies in the speech band. The filter at the east cable terminal-station'corresponding to lter 4 at the west station, is designedto suppress the control wavefrequencies f1 and f2 transmitted Vfrom the west station while passing other frequencies within the speech frequency band. The filter 13 in the circuit connecting the control wave sources 11 and 12 tothe ypath TA is designed to transmit the control wave frequencies `f1 and f2 While suppressing other frequencies. The corresponding nlter at the east terminal station is designed to pass the control wave frequencies .f3-.and f4 while suppressing other frequencies.

The control wave frequencies f1 and fz'generated by the sources 11 and 12, respectively, at the west cable terminal station, and the control wave frequencies f3 and *fr generated by the corresponding sourcesat the east station, vare so chosen that theyare notnormally to be found'siniultaneously in speech, in their harmonics or difference tones.

Thus, both control wave frequencies at each terminal may be below the lowest transmitted speech frequency and separated by greater than syllable frequency and less than fundamental speech frequencies, or` both maybe Ain the speech `band i1' properly chosen. One may be low in the :band

and the other high, not harmonically related, for, according to analyses of normal speech both low and high frequencies do not occur therein simultaneously. The chance of false operation of the receiving control circuits by speech will thus` be reduced although noise considerations might well iniiuence choice of frequencies. Control wave frequencies f1 and fz transmitted from the west station should be different frequencies from the control wave frequencies f3 and f4 transmitted from the east cable terminal station.

Fig. 2 shows a modified circuit arrangement which may be used in place of the portion of the receiving control circuit 14 shown within the dotdash box. It comprises two portions connected in parallel to the output of the selective network 15. One parallel portion comprisesv the filter 29 designed to pass the control wave frequency fa transmitted from the east cable terminal` and to suppress the controlY wave frequency f4 transmitted-from that terminal, the wave-controlled, relay-controlling device 30 (similar to the device 18) and the winding of mechanical relay 31 connected to the output of the control device 30 and arranged to be energized by the operation thereof. The other parallel portion comprises theillter 32 designed to pass the control wave frequency f4 and to suppress the control wave frequency f3 the wave-controlled relay-controlling device 33 and the winding of the mechanical relay 34 connected to the output of the device 33 and arranged to be energized by operation thereof. Relays 31 and Bloperate in response to the energization of their windings respectively to close the normally open switch contacts 35 and 36 in an energizing circuit for the windings of relays 19 and 20. When the switch contacts 35 and 36 are both closed, ther windings of relays 19 and 20 will be energized.

The operation of the system as a whole will now be described, referring to the west cable ter- It .will be assumed that the west subscriber as sociated with the west cableterminal station shown in the drawing Astar-ts to speak'before'the east subscriber associated with the east cable terminal station not shown. The west subscribers 's speech waves in the transmittingV path TA are divided between the input of the amplifying de-V vice l therein and the input of the control circuit 5 connected thereto. waves impressed upon the amplifying device l are amplified thereby and transmitted to the amplifying device 2 in the transmitting path TA which amplifies them stiil more. The portion of the west subscribers speech waves diverted into the control circuit 5 will operate the control device 6 therein causing the windings of relays 7 to l0 in the output thereof to be energized.

Y Relay '7 is designed so that it will operate in respcnse to the energization of its winding simul-V taneously with or slightly before the operation of -relay 8, and to release simultaneously withor slightly after relay 8 releases. The operation of relay '7 opens the switch contacts 37 in the input of the receiving path RA so as to disconnect the" receiving-amplifying device 3 from the cable TC. Relay 8 operates to close the normally open switch The portion of the speechl contacts' 23'in the output of transmitting path TA making that path operative to transmit from the output of the amplifyingdevice 3 into the cable TC. The control waves of frequencies ,f1 and f2 selected by the filter 13 from the output of sources l1 and 12, respectively, after amplification in the amplifying device 2 are then transmitted out over the cable TC along with the ampliedspeech waves in the output of the device 2. lRelay 9 is designed by any of the well known means to-be slow operating, so as to operate a small interval of time, t1, after the operation of relay 8,` to open switch contacts 25 to disconnect the-control wavey sources 11 and V-12 fromY the transmitting path TA. The time t1 should be equal to, or greater than the overall transmission time through the receiving control circuit 14. Relay 9 may be dispensed with in an alternative type of circuit, in which case the control wave frequencies will be continuously transmitted over the system as long as the speech waves are being transmitted thereover.` As this alternative method mig-htresult in some interference with the speech waves, the former method is probably preferable. y

- The inclusion 'of some means for delaying the transmission of speech waves to the cable TC with respect to the transmission vof the control waves thereto,such as an electrical delay circuit of well known design located in the transmitting path TA before the` point of connection of the sources 11` and l2 thereto, as indicated by the dotted 00x28 in the drawing, would make the system of the invention a courier system.

Relay 10 by any suitable means is designed to operate in response to thek operation of control device 6 by the speech waves from the path TA,

- slightly more than T seconds after relay 8 has operated to render the transmitting path TA op-- erative, where T is substantially equal to the overall transmission time of the cable TC between the west and east cable terminal stations. TheY relay 10 is also designed to have such small hangover as to release almost immediately with cessa tion ofthe supply of speech waves to the control device 6, or any desired hangover depending on the desired operation. Relay 10, then, will open the normally closed switch contacts 27 inthe receiving control circuit 14 substantially at the sance time* as the control waves f1 and f2 which have been previously transmitted over fthevcableVV TC, reach and seize-control ofthe switching apwhich will be described below.

y'The manner in which control of the transmitting and receivingpath and the associated control circuits at the east cable terminalstatio-n is paratus attheeastcable terminal in the manner obtained for Atheilrst talker associated with the` west station will now bedescribed and will be clear by reference to the similar apparatus at the minal station, the transmitting path TA thereat will be in its normal disabled condition, and the receiving path RA andthe receiving control circuit 14" will be in .their normally operative posiyso t tion. Then, the received'fwaves willr divide between the receiving pathRA and the input fol the receiving control circuit 14". f e Y The portion or the combined speechfand control waves diver ed into the receiving path RA will be ai'nplied by the amplifying device 3 therein and the ainpliedwaves will be divided between the input of the iilter 4 in the path BA and thevinput oi the control circuit 2l. Y The filter 4 will suppress the 'frequencies f1 and 'f2 -in the impressed wavesY and transmit the'rernfain-V ing frequencies inthe speech'waves to thecirout associated A'with the least subscriber. kThe portion of the amplified speech and ycontrol waves from the output of the amplifying device 3' diverted in thecontrol circuit 2l will operate the control f'device 22 therein causing the windings or the relays 19 and 20 inthe output thereof to be energized The deviceY 22 will be maintained in the operated condition bythe iin-` pressed speech waves when transmission of the controlwaves to the device 22 `ceases-due Ato Ythe operation of relay Q-at the west terminal station.` Relay i9 will-operate toopen the normally closed switch contacts 24 in the input hereby reventingany later linitiated'speech currents troni the subscriber getting into the control circuit 5 and causing false operation of the circuit. Relay 20 sirnultaneo'vusl'yl operates to open the normally closed contacts 26 in the circuit connecting the outputvoi the control device 6 to the winding-of relay l0 thus prevent ing `false operation thereafter of relayy l0 by any s eech waves of the east subscriber which might be stored in the transmitting `path TA or in the control circuit -5 at the Vtime relay i9" operates to disable the input of the path TA.

The west subscriber has new obtainedV cornpletecontrol of the talking circuits at both the weal and east terminal stations andthe east subscri er ill not bea le to break-in to transmit his own speech waves over the cable until there is a pause in the continuous transmission of Vthe west subscribers s l5 is designed to select the frequencies f1 and f2.

The frequencies f1 and f2 passed by the network l5 will be combined the ampliiler-de'tector 16'. Of the coc-.ibination products of the V frequencies fr and 'z in the output of the amplifier-detector 16, the diierence frequency f2-f1 or lsome other combination frequency will be selected by the .band-pass filter 17 and impressed on the control evlce i8 which will operate to `supply lenergizing current to the windings lel relays 19-` and'ZD which relays, above poiie-scdout, have been previously operated by control device 22 and maintained operated thereby in response to the speech waves received over the cable TC. ,I

1t is alp'rent then that for the case where the two subscribers Vat the opposite ends `of the sys@ tem do not start talkingv within ar time interval equal 'to the 'overall transmission time of the cable TC between the terminal stations, .withth'e circuit described; ie'Xclusive control or systern given to the first talker and the .second tall-rer by merely starting' to talk cannot seize 7.5 control of the circuit 'away from the iirst talker.

i the path TAk ech current long enough tov Vallow release of the relay 19 at' the east cable However; alter the rst talker gets control over both'eterminals in tliemanner described above, if the second talker persists in tryingto interrupt the iirst talker, he will succeed in gaining .control'oi-the'system`wnen the first talker pauses for only a short interval -o time -rnucn less than the time 'cf transmission'over t` ecable, as will be explained below. Y y l i 'It lwill `e a'ssur'ned thatrthe rst talker, the westsul'iscriber,A has obtained control of both terminals in the iznahnerdescribed above, and then, that'rth'e Ai'st subscriber pauses `for a time intervol ledualfto or reater--tli'an `the transmission time through the control circuit lll vat the west cable terminal station or the' corresponding control circuit' lll 'at the east cable 'terminal station,`the overall transmission time over each 'of those circuits to simplify `the explanation or" the operation or" the system beine assumed to be equal."v At the'boginning'loi this pause, thecon-v trol device G 'att1 westterininal station will release cai in elays 7 to 1) in its output to release also.` As stated above, although relay 10 at west terminal station is designed to operate witlia `tine lag of T seconds, it is designed Valsoltorelease la very short time aiter the cessY fion ci" speech waves tothe control evice 5.` The Vcircuits at .e west cable terminal station, therefore',` momentarily return to their norrnal conditionl as indicatedin the drawing with the Vtransrnitting path `TA disconnected from the cable,lthe receiving control circuit lll operative and 'the"rec'eiving` path RAl connected to tie cable. During this pause the control devices 18' and 22'v at the east cable terminal station will Aalso release duc to cessation of supply of inputcurrents thereto, allowing the windings of relays i9 and 20 Vconnected to their-outputs to be deenergized. The release or relay 19 closes the switch contacts 24 in the input or the transmitting path TA and the 'release of relay 2G causes the switchcontacts 26 in the inputk of relay l0 also Yto return to their normally closed condition. The east subscriber now, if he so desires, can get his speech waves through tothe input kof'the'control device 6 in control circuit 5 'at the east cable terminal station. The

resultant operation of control device 6 will en-v ergize the windings of relays 7 to l0 in'its output. Relay 7 immediately operates to open switch contacts 37 disconnecting the receivingamplifier v3 from the cable'TC. Relay 8 simultaneously operates to close switch contacts 2,3 connecting the transrnitting-arnpliier 2 to the cable TC and `allowing the amplified control waves of frequencies fg and f4 generated by the control wave sources 1l and l2 at the east terminal station after ainpliiication by the amplifying'device 2 to be transmitted out over the cabe TC along with the 'arni fied east subscribers speech waves. pproxinlately 'J1 seconds after the control waves f3 and fr been transmitted out over cable TC, where, as stated above,

the time interval t1 .sjedual to or greater than the overall transmission time of the control cir-v cuit l-l -at the east terminal station, relay 9' operates to disconnect the sources of control Waves f3 and fr iron/1 the transmitting path TA. Substantially T seconds after the control waves of frequencies jfs and fr have been transmitted out overthe cable' TC, relay l0 operates to open the normally closed switchfccntacts 27 in the receiving control vcircuit le. The east subscriber has now seizedoinplete .control oi the circuit at his own 'terminalawayirom the west subscriber.

Suppose at the time ofthe arrival of the east subscribers control waves f3 and fr and speech waves at the westcable terminal station over the cable TC, the west subscriber may have started to talk again and by again operating his control device 6 causes relays 7 and VV8 to respectively break his receiving path RA by opening the contacts 37 therein, and render operative his transmitting path TA by closing normally open contacts 23 therein, so as to transmit his control waves f1 and f2 along with his speech waves to the cable TC. However, this will not prevent the east subscribers currents from seizing control of the'circuits at the. west cable terminal station. Because'of the delay of T seconds in the operation of the relay 10 in response tol the west subscribers speech currents after the west subscribers control waves have been transmitted tothe cable dueto the design of that relay, the switch contacts 27 in the receiving control circuit 11i atthe westl cable terminal station are still in the closed condition and the east subscriber can get his control waves fs and f4 through to the ampliiier-detector 16.

The control waves of frequencies f3 and fr transmitted from the .east cable terminal station will be selected by the selective network l5 in the control circuit 14 and, the switch contacts 2'7 inthe output thereof being closed, will be transmitted to and combined in the amplierdetector V16. Of the combination products in the output of the amplifier-detector 16, the beat frequency )ii-f3 is selected by the band-pass lter 1'( and impressed on the input of the control device 18 causing its operation to energize the windings of relays 19 and 20. i

If the alternative circuit of Fig. 2 is used in the place of the portion of the control circuit 14 within the dot-dash box, the frequencies f3 and fr will be separated by the filters 29 and 32 and utilized to operate control devices 30 andv 32 to respectively energize relays 3l and 34. The resultant operation of relays 31 and 311 will close switch contacts 35 and 36 in the energizing circuit for relays 19 and 20 causing the operation of the latter relays.

Relay 19 will operate immediately to disable the input of the transmitting path TA by opening the switch contacts 24 therein and the relay 20 will simultaneously operate to disable the input of the control relay 10. The control device 6 in control circuit 5 on cessation of the supply ofthe west subscribers speech currents thereto due to the operationof relay 19 will release, causing the windings of relays 7 to 9 in .its output to be deenergized. The output of the transmitting path TA will therefore be again disabled by the opening or" its contacts 23, and the receiving path again rendered operative by the closing of its switch contacts 37.

The speech waves received over the cable TC from the east Vcable terminal, except for the portion reaching the point in pathrRA -where the switch contacts 37 are located before these contacts have closed, may therefore be transmittedl over the receiving-amplifying path RA tothe input amplifying device 3. These waves will be amplied by the amplifying device 3 land transmitted to the band-passfilter 4 in that path and to the input of the control device 22 in thereontrol circuit 21. The portion of the speech waves impressed on the control 'device' 22 will cause its operation to maintain Vrelays 20 and 19 en-` ergized; The band-pass lter 4 will transmit the subscribers speech currents except for the frequencies fs and f4 to the west subscriber.

From the'above description of the operation of the system it will be apparent that transmission lockouts when two subscribers at opposite ends of the system start talking about the same time are limited to the time required for currents to pass through the receiving control circuit 14, 14 at each terminal. The time of hangover of the relay l0 or l0', for disabling the receiving control circuit, if the same .at both terminals, determines how often each talker will `be subject to interruption. By changing the hangover oi' these relays so that they are different, the circuit may be biased as regards interruption toward one talkeror the other.

There will be some mutilation of speech in breaking a lockout with the circuit vdescribed since selectivity requirements for lters will not allow fast interruption through outgoing speech. The inclusion of delay of the proper amount to speech in the transmitting circuit at each terminal, between the input of the transmitting control circuit and the point in the transmitting circuit at which the sources of control waves are connected thereto, making the transmission oontrol a courier system, may reduce this mutila-V tion of speech appreciably, but will increase the time of transmission through the circuit.

All of the 'control relays in the system which relays.

Although in the particular embodiment of the invention described, mechanical relays have been shown for conditioningA the transmission paths by operating movable switches to close or open circuit these paths, the invention is not limited to the particular conditioning means shown. For example, within the scope of the invention the transmission paths may be effectively disabled or made operative by the insertion or re` moval of short-circuits vor loss networks in,A the pathsv controlled by v.f'ave-controlledV apparatus. Stationary devices having no movable elements may belemployed in place of the mechanical relays shown, for example, vacuum tube relays, such as disclosed in Crisson Patent 1,647,212,

issued November 1, 1927, or in article by C. A.V Beer and G. T. Evans in the Institute of P. O.

Electrical Engineers (London), vol. 20, pages 65 to 72 inclusive, published in 1920.

It is to be understood that the invention is not limited to the exact details of the circuits and the terminal stations illustrated and described as numerous modifications thereof may be made by persons skilled in the art without departing from the spirit and scope of the invention. For example, although in the system described only two .control currents are transmitted from each terminal station, a greater number of control currents may be used each of diiferent frequencies requiring slight changes in the design of the apparatus used at each terminal, so as to decrease the probability of false operation.V

What is claimed is:

1. Asignal transmission system comprising a signal transmission path supplied from a source of signals of a plurality of frequencies, wave-controlled, circuit-control apparatus at a control point along said path and adapted when operated to properly condition saidpath for transmitting said signals, means for transmitting over said path along with said signals, and under the con-l trol thereofla pluralityof control waves of different frequencies not normally found simul-l taneously in said signals, means for selecting thel control waves at said control point, means for combining the selected control waves, and means for selecting and utilizingr a combination wave for controlling said circuit-control. apparatus.

2.. A two-Way signal transmission system oom-` prising a two-waysignal transmission path connecting stationseach comprising wave-controlled, circuit-control apparatus for directionally controlling signal transmission in ther system, means at each station under control of the signals there initiated for transmitting over said path to another station, and along with said signals, a plurality of control waves of different frequencies and different for each station, the frequencies of the respective control waves at each station being chosen such that they are nctnormally found si-v multaneously in the signalstransrnitted between`r said stations, in their harmonics or difference frequencies, a control circuit at` each station connected to said path, said control circuit-comprising means for receiving and separating from the ivf,

received signals rthe control Waves from the other station, means for combining the separated control Wavesto fornnbeat waves, and means for selecting and utilizing one of said beat waves to operate the circuit-control apparatus at the station, and means at each station alsoresponsive to the signals there initiated for disabling the control circuit at the same station at substantially the same time as the control waves therefrom reach the other station.

3. A two-Way signal transmission system comprising a two-way signalV transmission path connecting stations each comprising a signal transmitter, a signal receiver and Wave-controlled, circuit-control apparatus which whenV operated dising chosen such that they are not normally found simultaneously in the signalstransrnitted between Y said stations, a normally operative, control circuit at eachstation connected" to saidvpath andl g comprising meansfor selecting the control Waves:

received over said tivo-way path from the other station, means for combining the, selected control waves to form beat waves and means for-selecting and' utilising one of said beat waves to operate the circuit-control.apparatus at the station, and means at each station responsive to the signals initiated inthe transmitter thereat for disabling the control*circuitr at the same station substantially at the time the control Waves transy ,mitted therefrom in advance of said' signals operate thecircuit-control apparatus at the other station'.

prising a two-way signal transmissionV path of limited transmission frequency range connecting stations each comprising a signal transmitter, a

signal receiver and wave-controlledV circuit-control means which when operated disables said. transmitter While allowing transmission from said spath. to said signal receiven, means at each station under control of the signals initiated in the transmitter thereat when operative for transmitting over said path to another station, and. inA advance of said signals, a plurality of short trains of control waves each of different frequencies and differentfor each station, the frequencies of the respective trains of control waves transmitted from each station being. chosen such as would not normally be found simultaneously in the signals transmitted betweenthe stations, in their harmonies or difference frequencies, a normally operative control circuit at each station connected to said path and comprising means for selecting the control waves received from said path, means for combining the selected Waves to form beat Wavesand means for utilizing one of the-beat Waves to operate initially 'said circuit-control apparatus at said other station, and means-for utilizing the signals received at said other station` over said pathifor maintaining the circuit-control apparatus in the operated condition initiated bythe preceding controlwaves during signal transmission between said. stations.

5. A signaltransmission system comprising a signal transmission path supplied from a source of telephonie waves, Wave-controlled, circuit-control apparatus at a controlpoint along said path and adapted when. operated. to properly condition saidpathfor the. transmission of. said signals, means for transmitting over said path and along with said signals` control waves of high frequency and control Waves of low frequency, said high. and low frequencies being within the frequency range of saidk telephonie waves and not harmonically related, means at said control point forseparating Waves of said high and low-frequencies from the other received Waves, means for combining the separated high and 10W frequency Waves,l and means for selecting and utilizing one of, the combination Waves for controlling` said circuit-control apparatus in proper` manner., y

6..A. terminal circuit for a two-Way signal transmission medium of .appreciable overall' transmission time, between terminals, comprising. a transmitting circuit and a receiving circuit connectedtosaid medium, means responsive to the initiation ofV signals in said .transmitting circuit,. and operative at a timethereafter substantially' edualto saidY overall transmission time of.v said medium, for disabling` said` receiving circuit, and means responsive toy the cessation of said' signals', in said transmitting circuit' and at a time thereafterk small comparison with said 'overall transmission time of said medium for rendering said receiving circuit' again operative.

7. A terminal circuit for a two-way signal transmission medium of appreciable over-all transmission time between terminals, comprising a normally disabled transmittingjcircuit and a receiving circuit connected to said medium; means responsive tothe initiation ofA signals inv said transmitting circuit for, rendering it opera-tiveto' transmit said` signals to said medium, meansalso responsiver to the initiation of :said signals ini said transmitting circuit and operative at a time after l said transmitting circuit has been rendered op- Li. A two-way signal transmission system com erative substantiallyV equalvto said over-all transmission tirne ofsaidy medium. for disablingsaidreceiving'circuit, and means-responsive substantially immediately to thecessation of said signals in saidtransmitting circuit for. again rendering. operative said receiving circuit.`

' 8. AtWo-Way Atelephone system comprising a tWo-waytransmission medium` connecting stations, each station comprising a control circuit, means at each station responsive to the initiation of speech Waves by a subscriber associated therewith for causing, inY addition to said speech Waves, control Waves to be transmitted over said medium to the other station, means also responsive to said speech Waves for eilectively disabling the control circuit at the same station substantially at the same time as the arrival of the control Waves at the other station, and for rendering the control circuit operative again substantially instantaneously with the cessation of said speech Waves at the transmitting station, ,the control circuit at each station when operative being selectively responsive to the control Waves received over said medium to disable the means at said station for transmitting speech waves and control Waves to said medium, and said control-circuit disabling means, while permitting the speech Waves transmitted over said medium from the subscriber associated with the other station to be received by the subscriber at the nrst station.

9. A two-way signal Wave transmission system comprising a two-Way signal transmission medium connecting stations, means normally preventing transmission of waves between said stations over said medium, a source of control energy at each station comprising frequencies different for each station and dirfering at each instant .from the signal frequencies, means at eacn station responsive to the initiation of signals thereat for causing transmission of said signals together with the control energy from the source at the same station over said medium to the other station, a normally operative control circuit at each station, means at each station also responsive to the signals initiated thereat and operative at a time after the transmission oi the control energy to said medium substantially equal to the overall transmission time oi said medium between said stations, to disable the control circuit at the same station, and adapted to release to render said control circuit again operative substantially simultaneously with cessation of the controlling signals, and means in said control circuit selectively responsive when said circuit is operative, to the control energy received over said medium from said other station to prevent transmission of Waves to said medium at the iirst station, and to disable the control-circuit disabling means.

l0. A two-Way signal transmission system compris-ing a two-way signal transmission medium connecting tvvo stations, each comprising a normally disabled transmitting circuit, a normally operative receiving circuit, a source of signals a source of continuous control energy of frequencies different for each station and outeide the signal frequency range, connected to said transmitting circuit, means responsive to als from sad source of signals for rendering said transmitting circuit operative to transmit signals and control energy to said medium for transmission thereover to said other station, a control circuit connected to said medium, means also responsive to the signals in said transmitting circuit for effectively disabling said control circuit at substantially the same time as the arrival of the control energy at the other station, and for rendering said control circuit operative again substantially instantaneously with the cessation of said signals in said transmitting circuit, and means in said control circuit and selectively responsive when that circuit is operative, to control energy received over said medium from said other station, to disable said transmitting circuit and said control-circuit disabling means.

l1. A terminal circuit for a two-Way transmission medium comprising a normally disabled transmitting circuit, a normally operative receiving circuit, a source oi signals and a source of continuous control energy oi frequencies outside the signal frequency range, connected to said transmitting circuit, control means responsive to signals supplied from said sourcek of signals for rendering said transmitting circuit operative to tra smit said signals and said control energy to said medium for transmission thereover, andl for disabling saidreceiving circuit, a control circuit operatively connected to said medium, means also responsive to the signals supplied to said transmitting circuit, for effectively disabling said control circuit at a time after the transmission of said control energy to said medium substantially equal to the overall transmission time of said medium between terminals and for rendering said control circuit operative again substantially instantaneously with cessation of said signals in said transmitting circuit, means in said control circuit and selectively responsive when that circuit is operative, to control energy received over said medium to disable said transmitting circuit and said control circuit disabling means, and a second control circuit connectedv to said receiving circuit and responsive when that circuit is operative, to Waves received over said medium to disable or maintain disabled said transmitting circuit and said control means.

12. A two-Way telephone system comprising a two-Way signal transmission medium connecting stations, each station comprising a control circuit, means at each station responsive to the initiation of speech waves by a subscriber associated therewith for causing, in addition to said speech Waves, control Waves to be transmitted over said medium tosaid other station, means also responsive to said speech waves for effectively disabling the control circuit at the same time as the arrival of the control Waves at the other station, and for rendering the control circuit operative again a given small time interval after the cessation of said speech Waves, said time interval being different for the two stations so as to bias the system as regards gaining control in iavor of one or the other subscriber, the control circuit at each station when operative being selectively responsive to the control waves received over said medium to disable the means at that station for transmitting speech Waves and control waves-to said medium or to the controlcircuit disabling means While permitting the speech Waves received over said medium from the subscriber associated with the other station to be received by the subscriber at the i'lrst station.

ANDREW C. NORWINE.

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